This invention relates to coal-fired boilers and more particularly to particulate emission control devices therefor.
Coal-fired boilers produce combustion gases rich in particulates. Mechanical collection devices of the cyclone-type are normally employed to remove the particulates from the gases and generally achieve about 94% removal at a cost of about $2.00 per pound of steam.
A cyclonic mechanical collector is a cluster of tubes through which effluent gas is passed. See FIG. 2. Each of these tubes, one of which is schematically portrayed in FIG. 3, contains within it a smaller tube, on which vanes are affixed which act very much like fan blades. As the effluent gases enter the collector, they pass between the inner and outer tubes, and the vanes impart to the entering gas a cyclonic, downward motion creating an outer vortex moving down the collection tube. The centrifugal force of this entering vortex forces particulate matter outward against the walls of the collection tube, where it proceeds downward, falling by gravity toward the hopper at the bottom of the tube. The swirling gas is tightened into a smaller vortex by gradually tapering the bottom portion of each tube, and near the bottom it reaches a point of equilibrium, reverses direction, and passes up through the center of the inner tube, helped along by suction from the induced draft fan, for expulsion through the stack. The particulate matter which has been forced outward by the cyclonic motion of the entering gas stream falls out of the gas stream at the point of equilibrium and is collected in a dust hopper at the bottom of the tube.
With respect to particulate greater than 10 microns in size, this design works very well. For very fine particulate, however, two factors combine to reduce the removal efficiency of the mechanical collector. First, the fine particulate is less susceptible to the centrifugal forces which propel the matter outward. Thus, as the gas stream moves down the tube, a relatively lower proportion of the fine particulate is thrust to the outside and a relatively greater proportion remains suspended in the gas stream as it reverses direction for expulsion through the stack. Second, the sudden reversal in direction of flow that occurs at the bottom of the collecting tube has a tendency to reentrain some of the fine particulate matter that has been propelled to the outside of the outer vortex.
For environmental reasons, it is considered desirable to reduce particulate emissions even further. More efficient devices such as full stream bag houses are capable of removing another 2% or 3%, but at a cost of approximately $12.00/lbs of steam. Likewise, electrostatic precipitators are capable of removing additional particulates, but also at significantly increased cost. In this regard, such full stream devices require considerable space for installation, require significant capital investment and have well known operational problems and associated costs. Still other systems involving wet scrubbing a fraction of the boiler effluent passing through the cyclone will reduce particulate emissions, but again at higher cost, higher maintenance and creation of an additional environmental problem (i.e., water treatment).
It is an object of the present invention to provide a relatively economical adaptation to cyclonic separators for reducing the particulates emitted from coal-fired boiler systems to more environmentally acceptable levels. This and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.